Heavy hydrocarbon deposits include oil sands and oil shale. Oil sand is a loose to consolidated sandstone or a porous carbonate rock, impregnted with a heavy asphaltic crude oil, too viscous to be produced by conventional methods. It is also known as tar sand or bituminous sand. Oil shale is a sedimentary rock containing solid, combustible organic matter in a mineral matrix known as marlstone. The oil in the shale is captured within kerogen, a complex organic polymer material consisting essentially of carbon, hydrogen, oxygen, sulphur and nitrogen. Kerogen is substantially insoluble, but decomposes to yield oil when heated.
Additional names given to oil shales include black shale, bituminous shale, carbonaceous shale, coaly shale, cannel shale, lignitic shale, torbanite, tasmanite, gas shale, organic shale, kerosine shale, coorongite, maharahu, kukersite, kerogen shale and algal shale. The oil content of shale (gallons per ton of rock) varies from a low-grade yielding 10 gallons per ton (about 4 percent by weight) to 26 gallons per ton (about 10 percent), medium-grade, to about 36 gallons per ton (about 14 percent) for high-grade shale.
Shale oil is produced from the organic matter (kerogen) in oil shale when the rock is heated. This heating process is known as retorting, and the rate at which oil is produced depends upon the temperature at which the shale is retorted. Retorting temperature affects the nature of the shale oil produced. Low retorting temperatures produce oils in which the parafin content is greater than the olefin contents; intermediate temperatures produce oils that are more olefinic; and high temperatures produce oils that are nearly completely aromatic, with little olefin or saturate content.
Destructive distillation occurs during primary chemical processing in which the oil shale is heated in an inert atmosphere at a temperature high enough for chemical decomposition. The principal off-products are gases containing carbon monoxide, hydrogen, hydrogen sulfide and ammonia, oils and water solutions of organic acids, alcohols and ammonium salts. Crude shale oil when subjected to destructive distillation undergoes a reduction in its viscosity and an increase in its hydrogen content. Preferably, the destructive distillation proceeds at about 400 degrees centigrade, in a range of 100-1,500 psi pressure, and preferably in an oxygen free atmosphere.
Shale oil has been recovered from carbonaceous deposits lying near the surface by mining, crushing and aboveground retorting. Recovery from deep carbonaceous formations has been accomplished by in-place processing, more commonly referred to as in situ retorting.
In situ retorting is carried out by initiating a combustion zone in the vicinity of an injection hole or well penetrating a carbonaceous formation, supplying oxygen or air to the combustion zone and permitting the combustion zone to migrate through the stratum by supplying pressurized air. By this method, the heat and products of combustion of a substantial portion of the burning carbonaceous material is forced out into contiguous portions of the stratum, sweeping or driving fluid carbonaceous materials toward a production well, thereby stimulating the production rate from the reservoir. Such practice is commonly referred to as fire flooding or thermal recovery.